1. Field of the Invention
The present invention relates to a toner used for dry electrophotography, for developing an electrostatic image in an image forming process such as electrophotography, electrostatic recording or electrostatic printing. It also relates to a two-component developer and an image forming method.
2. Related Background Art
It is well known to form a latent image on the surface of a photoconductive material through an electrostatic means and develop it.
For example, methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are known in the art. In general, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive substance and according to various means, and then the latent image is developed by causing colored resin particles or a toner to adhere onto the latent image to form a toner image. Subsequently, the toner image is transferred to a toner image support material such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor to produce a fixed image. Where the process employs a toner-image transfer step, the process is usually provided with a step of removing the toner remaining on a latent image bearing member.
As developing processes in which an electrostatic latent image is converted to a visible image by the use of a toner, known methods include the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these development processes, there is commonly used a fine powder obtained by mixing and dispersing a coloring agent in a thermoplastic resin, melt-kneading the dispersion, cooling the kneaded product, and then finely pulverizing the cooled product. As the thermoplastic resin, polystyrene resins are commonly used, and resins such as polyester resins, epoxy resins, acrylic resins and urethane resins are also used. Carbon black is widely used as a coloring agent of a non-magnetic toner. In the case of a magnetic toner, a black magnetic powder such as magnetic iron oxide is widely used. In the case of the two-component developer, the toner is usually used in admixture with carrier particles such as glass beads, iron powder or ferrite powder.
The toner image formed on a final copied image forming medium such as paper is fixed thereon by the action of heat, pressure or heat-and-pressure. In this fixing step, heat fixing and pressure fixing have been hitherto widely employed.
In recent years, there has been rapid progress in image forming apparatus such as copying machines, as from monochromatic copying to multi-color or full-color copying, where two-color copiers or full-color copiers are being studied and put into practical use.
In methods of forming color images by full-color electrophotography, substantially all colors can be reproduced usually using color toners comprised of a yellow toner, a magenta toner and a cyan toner corresponding to the three primary colors.
In such methods, light reflected from an original is passed through color separation light transmissive filters that are in complementary relations to the colors of toners, to form an electrostatic latent image on a photoconductive layer. Subsequently, developing and transfer steps are taken to adhere toner on a support material. These steps are successively repeated plural times, and toners are superposed with registration on the same support material, followed by fixing in one pass to give a final multi-color image or full-color image.
In the case of a developing system making use of the two-component developer comprised of a toner and a carrier, the toner is electrostatically charged as a result of its friction between it and the carrier, to have the desired electrostatic charges and charge polarity, and thus a latent image is developed by the toner with utilization of static attraction. Accordingly, in order to obtain a good toner image (a visible image), the toner must have a good triboelectric chargeability, which mainly depends on its relation to the carrier.
To resolve this issue, materials that constitute a developer have been studied for the purpose of achieving superior triboelectric chargeability, e.g., investigating carrier cores and carrier coating agents, finding an optimum coating weight, studying charge control agents or fluidity-providing agents added to toners, and improving binder resins for toners.
For example, Japanese Patent Publication No. 52-32256 proposes a technique of adding a charging aid such as electrostatically chargeable fine particles to a toner; Japanese Patent Application Laid-open No. 56-64352, a technique of adding to a developer a fine resin powder having a polarity reverse to that of a toner; and Japanese Patent Application Laid-open No. 61-160760, a technique of adding a fluorine-containing compound to a developer to give a stable triboelectric chargeability.
Another proposal is also seen in an example in which a toner is incorporated with resin particles with a polarity reverse to the triboelectric charge polarity of the toner. For example, Japanese Patent Application Laid-open No. 54-45135 and Japanese Patent Publication No. 52-32256 propose to add colorless resin particles having smaller particle diameters than those of a toner. These publications, however, report that the toner and the reverse-polarity resin particles are different in behavior from each other, where the toner adheres to the latent image portion and the reverse-polarity resin particles adhere to the background portion when development is carried out. This means that the reverse-polarity resin particles promote the electrostatic charging of toners.
Japanese Patent Application Laid-open No. 1-113767 also proposes to use silica and organic resin particles at the same time. The silica and organic resin particles are used for the purpose of weakening the adhesion between a drum and a toner.
Japanese Patent Publication No. 2-3172 (U.S. Pat. No. 4,943,505) proposes a system wherein a toner and organic resin particles are used in mixture so that the electrostatic charging of toners is not deteriorated.
Various means are also proposed as a method in which the additive such as the charging aid as mentioned above and a toner are mixed. For example, it is common to use a method in which the charging aid is caused to adhere to the surface of toner particles by the action of an electrostatic force or the van der Waals' force, where a stirrer or a mixing machine is used as a means therefor. In such a method, however, it is not easy to uniformly disperse the additive to the toner particle surfaces, and is not easy to prevent agglomerates of the additive from being present in a free state in a developer. This tendency becomes more remarkable as the additive such as the charging aid has a larger specific resistance and the additive has a smaller particle diameter. The presence of a large quantity of agglomerates of the additive in a free state in a developer may affect the performance necessary for the developer. For example, the quantity of triboelectricity of the toner may become unstable to cause non-uniform image density, tending to give a foggy toner image.
When copies are continuously taken on a large number of sheets, there is a problem that the content of the charging aid may change to make it difficult to maintain the initial toner image quality.
As another method of addition, there is a method in which the charging aid, etc. is initially added together with a binder resin and a coloring agent when a toner or colored resin particles are prepared. It, however, is not easy to control the quantity of the charging aid, etc. added or the quantity in which it is dispersed to the toner particle surfaces, because it is not easy for the charge control agent to be uniformly dispersed, and also because those agents substantially contributing the chargeability are only those present near the toner particle surfaces and the charging aid or charge control agent present in the interior of a particle does not contribute to the chargeability. When toners are obtained in such a method, the toners tend to have an unstable quantity of triboelectricity. Thus, it is not easy to obtain a developer that can satisfy the development performances stated above.
Moreover, in recent years, there is an increasing demand for achieving a more detailed image and a higher image quality in copiers and printers. In the related technical fields, it has been attempted to achieve a higher image quality by making toner particle diameter smaller. As the toner particle diameter is made smaller, the surface area per unit weight of a toner increases. This tends to increase charges per unit weight of the toner, tending to cause deterioration of durability in the running of a large number of sheets. In addition, because of a large quantity of charges of the toner, toner particles may strongly adhere to one another to bring about a decrease in fluidity, tending to cause problems with stability in toner feeding and in providing triboelectricity to the toner fed.
In the case of color toners with chromatic colors, toner particles have no portions from which charges may leak, since they contain no magnetic material or conductive material such as carbon black. This tends to bring about an increase in charges. This tendency is marked particularly when a polyester type binder having a high charging performance is used in the toners.
The color toners are strongly desired to have the following properties.
(1) In order for color reproduction not to be hindered by a fixed toner because of irregular reflection of light, toner particles are required to be brought into a substantially completely molten state and deformed to such an extent that their original forms can not be recognized. PA1 (2) The color toners must be transparent so that an upper toner layer may not interfere with the color tone of a lower layer having a different color tone. PA1 (3) All color toners must have well balanced hues and spectral reflection characteristics, and sufficient chroma. PA1 said powdery additive comprising organic resin particles-(B) having peaks respectively in a region of particle diameters of 20 m.mu. to 200 m.mu. and a region of particle diameters of 300 m.mu. to 800 m.mu. in their particle size distribution, and the larger-diameter particles included in the region of particle diameters of 300 m.mu. to 800 m.mu. being contained in an amount of from 2% by weight to 20% by weight. PA1 said toner comprising colored resin particles-(A) containing a coloring agent or a magnetic powder, and a powdery additive; PA1 said powdery additive comprising organic resin particles-(B) having peaks respectively in a region of particle diameters of 20 m.mu. to 200 m.mu. and a region of particle diameters of 300 m.mu. to 800 m.mu. in their particle size distribution, and the larger-diameter particles included in the region of particle diameters of 300 m.mu. to 800 m.mu. being contained in an amount of from 2% by weight to 20% by weight. PA1 forming a toner layer on a developer carrying member by means of a coating blade; PA1 forming a developing zone between said developer carrying member and a latent image bearing member opposingly provided thereto; PA1 while applying a bias voltage across said developer carrying member and said latent image bearing member, developing a latent image formed on said latent image bearing member by the use of a toner of the toner layer formed on said developer carrying member, to form a toner image; and PA1 transferring said toner image to a transfer medium; PA1 said toner comprising colored resin particles-(A) containing a coloring agent or a magnetic powder, and a powdery additive; PA1 said powdery additive comprising organic resin particles-(B) having peaks respectively in a region of particle diameters of 20 m.mu. to 200 m.mu. and a region of particle diameters of 300 m.mu. to 800 m.mu. in their particle size distribution, and the larger-diameter particles included in the region of particle diameters of 300 m.mu. to 800 m.mu. being contained in an amount of from 2% by weight to 20% by weight.
Nowadays, polyester resins are widely used as binder resins for color toners. Toners containing polyester resins commonly tend to be affected by temperature and humidity, and tend to cause problems by creating an excess quantity of triboelectricity in a low humidity environment and in developing insufficient quantity of triboelectricity in a high humidity environment. Thus, it has been sought to provide an improved color toners and developers capable of having a stable quantity of triboelectricity.